1. Field of the Invention
The present invention relates generally to a shock absorber used for suspension systems of an automotive vehicle for absorbing vibration energy and more specifically to a shock absorber in which damping force can be adjusted automatically according to vehicle speed or manually according to the driver's preference.
2. Description of the Prior Art
When an automotive vehicle transverses a rough road, springs in the vehicle suspension system compress and expand to absorb shock. However, since the springs continue to vibrate until they return to their original state, the spring vibration reduces the road-holding ability and riding comfort of the vehicle. A shock absorber is used dampen on the spring vibration for insuring better road holding capability and better riding comfort. To obtain better riding comfort, a shock absorber generally provides greater damping action when extended than when compressed. This action is achieved by the use of valves which change the flow of fluid filled within the shock absorber. This double-acting shock absorber, which provides damping action during both extension and compression, is presently in common use on vehicles. Further, the faster the working speed of a piston employed in the shock absorber, the greater the damping force or damping action of the shock absorber. If the damping force is too great, a hard ride is obtained; if too soft, a soft ride is obtained.
In the conventional shock absorber, however, since the damping forces during both extenion and compression are usually fixed, the road-holding ability, the riding comfort, or the steering-wheel manipulability vary according to vehicle speed or road harshness.
To overcome the above-mentioned problems, there has been proposed a variable-damping-force hydraulic shock absorber provided with a motor, an adjuster, etc. in which part of working fluid within the upper and lower chambers is by-passed during operation through one of a plurality of orifices of various diameters selected by an orifice adjuster rotated with a motor. That is to say, since part of the amount of the working fluid through the upper and lower piston valves is diminished, a smaller damping force diminished from the conventional damping-force obtained by working fluid through only the valves can be obtained. In this case, the adjuster is controlled in response to a feedback signal detected by an angular position sensor attached thereto.
In such a prior-art variable-damping-force hydraulic shock absorber as described above, since part of working fluid displaceable passed between the two chambers is by-passed through the same orifice selected by the adjuster during both extension and compression, a variable amount of the damping force obtained in the extension mode of the piston is the same as that obtained in the compression mode, thus raising a further problem in that it is difficult to satisfy the above-mentioned request such that a shock absorber must provide greater damping action when extended than when compressed. In this connection, greater damping action during extension serves to improve the road-holding ability; while smaller damping action during compression serves to improve the riding comfort.
In shock absorbers for automotive vehicles, the total length of the absorber must be as short as possible from the standpoint of mounting space within the vehicle body, but the stroke of the piston rod must be as long as possible to absorb vibration energy (product of damping force and stroke). In the conventional variable-damping-force shock absorber, however, since additional space to mount various elements (provided with orifices through which the fluid is selectively by-passed) for adjusting damping force is inevitably required, the stroke of the piston rod becomes relatively short in the case where the total length of the shock absorber is fixed; that is, there exists another problem in that the stroke of the piston rod is not sufficiently long and therefore vibration energy is not sufficiently absorbed.